In integrated circuits (ICs), especially in power ICs, power dissipation may cause the device to reach relatively high temperatures. In order to avoid degradations phenomena or even worse destructive failures of ICs, because of excessive temperature, it is often necessary to integrate a dedicated protection circuit capable of "switching-off" at least a power output portion of the integrated circuit wherein primarily power dissipation occurs, whenever a condition of risk is reached. Basically, such a thermal protection circuit must have a precise triggering threshold, in terms of temperature reached by the integrated circuit, and a relatively small area requirement, for obvious economic reasons.
Very many forms of thermal protection circuits are known in the art. Basically the desired function of the circuit is achieved by integrating a component (typically a diode) having a known temperature characteristic and using it as a temperature sensor. By comparing the temperature dependent voltage across the "sensor" with a reference voltage that is stable in temperature, a desired temperature triggering threshold can be implemented. Almost exclusively, a temperature stable, reference .voltage is derived from a circuit which is commonly known as "BANDGAP" circuit. In practice, a fraction of the so-called bandgap voltage of the semiconductor is used as a stable reference in terms of temperature.
A thermal protection circuit, made according to a known approach is depicted in FIG. 1. When the voltage (V.sup.-) applied to a first input of the comparator C1 equals the voltage (V.sup.+) applied to the other input of the comparator C1, a desired transition of the signal present on an output node (OUT) occurs. The triggering temperature T may be determined as follows: ##EQU1## where V.sub.BE is the base-emitter voltage of transistor Q.sub.2, I.sub.c represents the current given by the difference between the base-emitter voltages of transistors Q.sub.1 and Q.sub.2 divided by the resistance of R.sub.1, and the term I.sub.s represents the saturation reverse current of the transistors.
By simplifying and resolving for T: ##EQU2##
The circuit at the left of the dash line in FIG. 1 is a so-called bandgap circuit. The temperature independent voltage that is produced on the so-identified "BANDGAP" node, is divided by a precision voltage divider R.sub.7 /R.sub.8 and compared with the voltage present across the diode Q344. A current substantially identical to the current flowing through the bandgap circuit is forced through the diode Q.sub.344 by the transistor Q.sub.9. At room temperature, the voltage on the inverting input (-) of the comparator C1 is lower than the voltage on the noninverting input (+). With an increase of the temperature, the voltage across the diode Q.sub.344 decreases with a known law. Therefore, at a certain temperature, the voltage across the diode Q.sub.344 will become lower than the voltage that is applied to the inverting input (-), thus making the comparator C1 change state.
As may be observed from the circuit analysis shown above, the precision of the triggering temperature is directly tied to the value of the resistance R.sub.1, which determines the current through the diode Q.sub.344, and to the current I.sub.s, which determines the V.sub.BE of the diode.
Another known circuit is shown in FIG. 2. The triggering temperature T may be derived as follows: ##EQU3## where V.sub.BE is the base-emitter voltage of transistor Q.sub.2, I.sub.c represents the current given by the difference between the base-emitter voltages of transistors Q.sub.1 and Q.sub.2 divided by the resistance of R.sub.1, and the term I.sub.s represents the saturation reverse current of the transistors.
By simplifying and resolving for T: ##EQU4##
Differently from the circuit of FIG. 1, in this other circuit, a fraction of the bandgap voltage is compared with a voltage that is proportional to the difference between the V.sub.BE of a first transistor Q.sub.3 and of a second transistor Q.sub.4 of the bandgap circuit. This voltage difference increases with temperature while the bandgap voltage remains stable. Therefore, at the temperature indicated by the second equation of the analysis shown above, the comparator C1 will change state.
Also in this case, the precision of the triggering temperature is tied directly to the value of R1 and to the level of the current I.sub.s.
As may be observed, the known circuits comprise a precise voltage divider (R.sub.7 /R.sub.8) for dividing the bandgap voltage produced by the homonymous circuit, through which a temperature-stable reference voltage is derived. Moreover, the known circuits employ a comparator (C1) to the inputs of which the temperature independent reference voltage derived from the bandgap circuit and a temperature dependent voltage are applied.